First you must find out what your interface name is.

To do that just run ip address from the Terminal.

On my machine it is eno1 which can be found on the first line:

ip address

returns:

2: eno1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 90:b1:1c:aa:bb:cc brd ff:ff:ff:ff:ff:ff
    inet 10.1.2.16/24 brd 10.1.2.255 scope global eno1
       valid_lft forever preferred_lft forever
    inet6 fe80::5cd1:3ee8:c461:6f12/64 scope link noprefixroute 
       valid_lft forever preferred_lft forever

Edit the file /etc/netplan/01-network-manager-all.yaml and make it look like this for a static IP address assignment:

/etc/netplan/01-network-manager-all.yaml

# Let NetworkManager manage all devices on this system
network:
  version: 2
  renderer: NetworkManager
  ethernets:
    eno1:
      renderer: networkd
      match:
        name: eno1
      addresses: [10.1.2.16/24]
      gateway4: 10.1.2.1
      nameservers:
        search: [example.com]
        addresses: [10.1.2.10]

This tells netplan to use networkd on the interface eno1 instead of NetworkManager.